"The change has been dramatic in the last 20 years with large increases in all championships both in the southern and northern hemisphere," Toussaint said.

"The increase in both forwards and backs has been around four to five centimeters over the past 20 years and an increase in 12 kilograms. It has been a large change," he added.

These muscle-bound physiques have broadly coincided with the advent of professionalism (in 1995) and have made size a key determinant of a team's fortunes. Bigger, it seems, is better, according to research led by Toussaint's INSEP colleague, Adrien Sedeaud.

In a study published in the British Journal of Sports Medicine in 2012, Sedeaud looked at the height and weight of more than 2,500 rugby players at six World Cups from 1987 to 2007.

What he found was that the teams which performed the best had the tallest backs and the heaviest forwards.

Other factors also came into play -- greater collective experience is also important, Sedeaud concluded -- but it was notable that amid all the complexities of measuring performance that things like player weight and height were so obviously playing a prominent part.

This supersizing of sportsmen hasn't been limited to rugby players, it's evident in many other elite-level sport. But why?

"These changes have both to do with the (rising) mean height of the population, but at the same time the knowledge of training, conditioning and the hours spent training and in recovery. Many of these things have played a small role in the changes," Toussaint said.

For tennis, the men increased by seven centimeters in 50 years and the women increased by 10 cm in the same years.Jean-Francois Toussaint, INSEP

Increased use of anabolic steroids may also be a factor, he thinks, although in rugby's case the incidence of positive tests is low compared with some other sports. In his study, Sedeaud points to the World Anti-Doping Agency's 2009 annual report which identified 39 positive samples out of 5,725 tests.

Non-contact sports like tennis and swimming have also seen the same upward curve in recent decades.

"For tennis, the men increased by seven centimeters in 50 years and the women increased by 10 cm in the same years. Both sexes have seen the same trend," Toussaint said.

But there are already signs that these growth spurts may be starting to tail off.

"We are seeing places where plateaus have been reached already or are close to being reached -- for example the height," Toussaint said.